The invention relates to a method of manufacturing contact structures wherein one or more contact members having multiple layers of contact material are mounted on a carrier strap.
Contact members are needed in large amounts. Their manufacture therefore needs to be largely automated. Since, furthermore, the materials of which contact structures consist is generally quite valuable, the contact structures should be so composed that the valuable material is used only for the contact surface areas whereas the remainder of the contact structure consists of less valuable, that is, less expensive, materials. In order to make the contact structures relatively inexpensive, contact structures are, in many cases, bimetallic, especially those used in high power applications. Such bimetallic contact structures are usually supplied as contact rivets with rivet stems and rivet base plates consisting of copper and the contact area consisting of silver or a silver alloy. Such contact rivets are manufactured by welding a wire of copper or a copper alloy face-to-face together with a silver wire of corresponding diameter, cutting the wire and stamping the cut section into a rivet. Such contact members, although quite common in power supply systems, are not usable in connection with low voltage systems since the resistance at the bimetal interface of the silver or silver alloy contact areas is too high for low voltages or low currents. For such applications it is generally necessary to use noble metal contact areas, preferably gold contact structures or contact structures plated with gold. For economic reasons, however, the noble metal contact layers must have only limited thickness and can therefore not be manufactured in the same manner as the bimetallic contact structures previously described. For further savings in the use of noble metals, multiple contact metal layers preferably with a top layer of gold are sometimes utilized. But the manufacturing method described herebefore cannot be utilized for this kind of contact structure either.
It is also known in the art to make contact structures out of a contact strip having several layers of contact material disposed on a base strip of a relatively inexpensive material. Dice- or rod-shaped sections are cut from the strip and then stamped or shaped and press fitted or riveted into corresponding openings in a carrier strap. A disadvantage of this method, however, is that the contact material layers are heavily stressed by the subsequently necessary mechanical deformations whereby the desired layered structure will not normally be maintained. It has therefore also been tried to produce contact structures without first deforming the contact strip material. For that purpose, contact sections were cut from the contact strip and inserted in corresponding openings punched into a carrier strap into which they were stamped. However, this process also applies substantial mechanical stress to the contact layers.
Further, especially in order to save contact material, the contact material has been cut in such a manner as to produce wedge-shaped sections providing for members of generally eight edges. But naturally, also such a contact member is mechanically heavily stressed when being stamped into a carrier strap.
The heavy mechanical stresses referred to above are caused mainly by the fact that the thickness of the cut contact members is essentially constant over the whole contact member so that, consequently, a relatively large amount of material needs to be worked by application of correspondingly high pressures--in contrast to the contact rivets referred to above.
Taking into consideration the need for automated manufacturing, a method of producing contact members should therefore have the following particulars:
1. It should permit automatic mounting of the contact members onto a carrier strap.
2. Application of the contact members to the contact strip should generate the least possible amount of stresses, that is, the contact members should essentially not be deformed.
3. The contact members should have very thin noble metal contact surface layers of an even thickness of about 0.2.mu..